Position tracking and imaging system for use in medical applications

ABSTRACT

A system is disclosed for monitoring the position of a medical instrument with respect to a patient&#39;s body and for displaying at least one of a plurality of prerecorded images of said body responsive to the position of said medical instrument. In one, embodiment the system includes a reference unit secured from movement with respect to the patient&#39;s body such that said reference unit is substantially immobile with respect to a target operation site. The system also includes a remote unit for attachment to the medical instrument. A field generator may be associated with one of the units for generating a position characteristic field in an area including the target operation site. One or more field sensors may be associated with either of the units responsive to the presence of the position characteristic field for producing one or more sensor output signals representative of said sensed field. A position detector in communication with the sensor output signal produces position data representative of the position of the remote unit with respect to the reference unit. An output display in communication with the position detector displays at least one of the prerecorded images responsive to the position data.

BACKGROUND OF THE INVENTION

[0001] The invention relates to computer assisted medical surgery and inparticular relates to systems for displaying prerecorded visual imagesduring surgical operations.

[0002] Presently available medical imaging techniques such as CAT(Computerized Axial Tomography), MRI (Magnetic Resonance Imaging), andPET (Position Emission Tomography), are known to be helpful not only fordiagnostic purposes, but also for providing assistance during surgery.Prerecorded images may be displayed during surgical operations toprovide the surgeon with illustrative reference mappings of pertinentportions of a patient's body.

[0003] Tracking systems for monitoring the position of a medicalinstrument have also been developed for use with image display systems.Generally, as the surgeon moves the medical instrument with respect tothe patient's body, associated prerecorded images are displayedresponsive to the movement of the instrument. Such tracking systemstypically involve either the use of a passive articulated arm attachedto the medical instrument, optical detection or ultrasonic detection.

[0004] Tracking systems using a passive articulated mechanical armattached to a medical instrument are disclosed in U.S. Pat. Nos.5,186,174 and 5,230,623. Generally, as the surgeon moves the surgicalinstrument with respect to the patient's body, micro recorders at thejoints of the articulated arm record the respective amounts of movementof each arm member. The outputs of the micro recorders are processed andthe position of the medical instrument with respect to the base of thearticulated arm is thereby monitored. One or more prerecorded images arethen displayed responsive to the movement of the surgical instrument.Such articulated arm tracking systems, however, require that theinstrument be attached to a cumbersome mechanical arm. Also, althoughfree movement of the tip of the arm in three dimensional space may betheoretically possible, the surgeon might experience difficultypositioning the instrument at certain locations and in desiredorientations within the body.

[0005] Tracking systems using optical detection (video cameras and/orCCDs (Charge Coupled Devices)) have been proposed for monitoring theposition of a medical instrument with respect to a reference unit asmentioned in U.S. Pat. No. 5,230,623. Such systems, however, requirethat the reference unit and the instrument both be within the view ofthe camera. This not only limits the movement of the surgical staff, butalso requires that at least a portion of the medical instrument remainoutside the patient's body.

[0006] Tracking systems using ultrasonic detection are generallydisclosed in U.S. Pat. No. 5,230,623. Such systems, however, aredisclosed to be used in a fashion similar to optical detection, i.e.,triangulation of transmitted signals. The transmitted signals are sentfrom one or more senders to associated receiver(s), and the distancestravelled by the signals are determined from either timing or amplitudechanges. Again, the transmission path must remain unobstructed.

[0007] A further shortcoming common to each of the above trackingsystems is that the patient must not move during the operation. Althoughthe patient is likely to be generally anesthetized, the patient's bodymay be inadvertently moved by the surgical staff, or the surgeon maywant to move the body for better positioning. If the body is moved afterthe tracking system has been initialized, then the tracking will bemisaligned.

[0008] There is a need therefore for a system for monitoring theposition of a medical instrument with respect to a patient's body thatavoids the shortcomings of present devices. Specifically, there is aneed for a tracking system that permits a medical instrument to bestructurally unattached to a reference unit, yet capable of fullyentering into the body without loss of position monitoring.

[0009] There is also a need for a tracking system that monitors theposition of the patient during surgical operations.

[0010] There is also a need for a tracking system that includes areference unit that may be easily removed from and accuratelyrepositioned on a patient in precisely the same position. There isfurther a need for a position monitoring device that does not obstructthe operating space of the surgeon.

SUMMARY OF THE INVENTION

[0011] The invention relates to a system for monitoring the position ofa medical instrument with respect to a patient's body and for displayingat least one of a plurality of prerecorded images of the body responsiveto the position of the medical instrument. The system includes areference unit, a remote unit, a position characteristic fieldgenerator, a field sensor, a position detection unit and an outputdisplay.

[0012] In one embodiment, the reference unit is secured from movementwith respect to at least a portion of the patient's body such that thereference unit is substantially immobile with respect to a targetoperation site. The remote unit is attached to the medical instrument.The field generator is associated with one of the reference or remoteunits and generates a position characteristic field, such as amultiplexed magnetic field, in an area including the target operationsite. The field sensor is associated with the other of the reference orremote units and is responsive to the presence of the field forproducing a sensor output signal representative of the sensed field.

[0013] The position detection unit is in communication with the sensoroutput signal and produces position data representative of the positionof the remote unit with respect to the reference unit. The outputdisplay unit is in communication with the position detection unit fordisplaying at least one of the prerecorded images responsive to theposition data.

[0014] The system further may include a registration unit incommunication with a storage unit and the position data. The storageunit stores the plurality of prerecorded images of the body. Eachprerecorded image is representative of a planar region within the bodysuch that the plurality of planar regions represented by the prerecordedimages define a first coordinate system. The registration unitcorrelates the position data of a second coordinate system (as definedby the position detection unit) with the plurality of prerecorded imagesof the first coordinate system, and identifies a desired prerecordedimage associated with the position of the remote unit with respect tothe patient's body.

[0015] The invention also relates to a reference unit that is attachableto a patient's head, and a medical instrument, such as an aspiratingdevice, that is adapted to removably receive a position detection unit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The following detailed description of the invention may befurther understood with reference to the accompanying drawings in which:

[0017]FIG. 1 is a diagrammatic view of a system of an embodiment of theinvention;

[0018]FIG. 2 is a front view of the headset unit shown in FIG. 1;

[0019]FIG. 3 is a side view of the headset unit shown in FIG. 1 takenalong line 3-3 of FIG. 2;

[0020]FIG. 4 is a rear view of a portion of the headset shown in FIG. 1taken along line 4-4 of FIG. 3;

[0021]FIG. 5 is an exploded side view of the surgical instrument andremote sensor shown in FIG. 1;

[0022]FIG. 6 is an end view of the assembled surgical instrument andsensor shown in FIG. 1 taken along line 6-6 of FIG. 5;

[0023]FIG. 7 is a diagrammatic view of an alternate embodiment of theinvention;

[0024]FIGS. 8 and 9 are diagrammatic views of image recording andregistration operations of the invention; and

[0025] FIGS. 10-13 are diagrammatic views of further embodiments of theinvention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0026] As shown in FIG. 1, a system 10 of the invention includes aheadset 12 mounted on a patient 14, a medical instrument 16, a controlsystem 18 and a display 20. The control system 18 includes a positiondetection unit 22, a registration unit 24, and an image storage unit 26.

[0027] The image storage unit 26 stores sets of prerecorded images suchas CAT, MRI or PET scan images. Each set of images may be taken along,for example, coronal, sagittal or axial directions. As shown in FIG. 1,the display 20 shows three images, a coronal image 21 a, a sagittalimage 21 b, and an axial image 21 c. Text information may also bedisplayed as shown at 21 d in FIG. 1.

[0028] As further shown in FIGS. 2-4, the headset 12 includes two earmounts 28 on side members 30, and a nose bridge mount 32 on a centermember 34. The headset 12 should be made of a resilient plastic suchthat it may be snugly attached to a patient's head, and may be providedin a variety of sizes. A primary objective of the headset is to providea reference unit that may be easily attached to and removed from apatient's head wherein the headset may be repeatedly reattached inexactly the same place with a high degree of accuracy. In otherembodiments, the side members 30 of the headset 12 may be rotationallyattached to one another and the ear mounts 28 may be biased toward oneanother. Further, the center member 34 may be rotatable with respect tothe side members 30 and biased toward the ear mounts 28 as well.

[0029] The headset 12 shown in FIGS. 1-4 also includes a reference unit36 connected to the position detection unit 22 via communication lines38. The reference unit 36 may be releasably attached to the headset 12by conventional clamp or fastening means. In one embodiment thereference unit 36 may include a position characteristic field generatorcapable of generating a multidirectional field in three dimensions andmay involve the use of either electromagnetic or ultrasonic waves. Theposition characteristic field differs from the transmit/receivetriangulation system, in part, because it does not rely on thecomparison of one transmitted signal with another as does triangulation.This permits the path between the field generator and the remote sensorto be obstructed by materials that do not significantly alter thegenerated field. For example, the position of the medical instrumentcould be identified even when the instrument is within the patient'sbody when the generated field in a magnetic field. Additionally, thereference unit may also include a reference sensor 37 to provideverification of proper system operation.

[0030] In the present embodiment the field generator includes threeorthogonally disposed magnetic dipoles (e.g., current loops orelectromagnets), and the orthogonally disposed magnetic fields generatedby each of the three dipoles are mutually distinguishable from oneanother (e.g., via either phase, frequency, or time divisionmultiplexing). The near-field characteristics of the multiplexedmagnetic fields may be relied upon for position detection, for exampleas generally described in U.S. Pat. No. 4,054,881. Since the presence ofmagnetic material might interfere with the magnetic fields thesematerials are to be avoided in such an electromagnetic system. Inalternate embodiments the field generator may be located somewhere otherthan on the headset and the headset may include two field sensors 36,37. When the distance between the sensors 36, 37 is known, the secondsensor acts as a backup or reference check for monitoring the properoperation of the system. If the sensed fields are inconsistent then anerror signal is displayed and/or sounded.

[0031] In other embodiments the headset 12 may be employed in systemsbased on the triangulation of signals where the reference unit 36includes one or more signal transmitters and/or one or more signalreceivers. In such a triangulation system, position detection isachieved by comparing certain characteristics of one transmitted signalwith those of a second transmitted signal to determine the relativedistances travelled. The transmitted signals may be electromagnetic(e.g., radio, laser light or light emitting diodes) or may beultrasonic. The position of the patient's head with respect to thesurgical instrument may thereby be monitored.

[0032] As shown in FIGS. 5 and 6 the medical instrument 16 may be anaspirating device adapted to removably receive a remote sensor 40 fordetecting, for example, the field generated by the positioncharacteristic field generator. The sensor 40 may be held inside theinstrument 16 by force fit sizing or through the use of a resilient snapmember in the wall opening 42. Since an aspirating device is commonlyused in most surgical operations, incorporating the remote sensor intothe aspirating device provides the surgeon with a convenient positiondetection device that does not clutter the operation site withunnecessary items. The instrument 16 may further include a second backupfield sensor 41 for system error detection as discussed above withreference to the sensor 37.

[0033] The remote sensors 40, 41 are removable from the aspiratingdevice and may be interchangeably inserted into any of a variety ofspecially adapted surgical instruments. In the illustrated embodiment,the remote sensors 40, 41 are received through an opening 42 in theproximal end of the instrument 16, and are connected to the positiondetection unit 22 via communication lines 44. The sensors 40, 41 mayalso each include three orthogonally disposed dipole sensing elementsfor detecting the presence of the field generated by the fieldgenerator. For example, in one embodiment, the field generator and thesensors each include three orthogonally disposed electrical wire loops.The generator produces an alternating current through one generator loopat a time thus generating a time division multiplexed alternatingelectromagnetic field. The sensor loop signals are each processed insynchronous timing with the generator loops to produce outputsresponsive to each respective alternating electromagnetic field.

[0034] The distal end of the instrument 16 includes a rigid aspiratingtube 46 having a flared tip 48. The position of the tip 48 with respectto the center of the remote sensor 40 is a known constant and may beeasily seen by the surgeon during surgery. The aspirating tube 46 is influid communication with an aspirating catheter 50 through the proximalend of the instrument 16 via internal channel 52 and a connector element54. The aspirating catheter 50 (shown in FIG. 1) is connected to avacuum aspirating unit (not shown).

[0035] In operation, the position detection unit monitors the positionof the medical instrument 16 with respect to the reference unit 36. Theregistration unit 24 correlates the changes in position of theinstrument 16 with the spacial orientation of the stored images. As thesurgeon moves the medical instrument 16, images appear on the display 20responsive to the position of the medical instrument 16. This permitsthe surgeon to always have available the coronal, sagittal, and axialviews associated with the precise location of the tip 48 of theinstrument 16 regardless of whether the tip 48 is inside of the patient14. Moreover, since the field generator is attached to the patient'shead, the patient is free to be moved without loss of the trackingcapabilities. The display 20 may further identify the location of thetip 48 on each of the displayed images as shown at 56 in FIG. 1. Inother embodiments the orientation of the aspirating tube 46 may also beidentified on the displayed images. In further embodiments, a threedimensional composite image may be displayed based on the prerecordedimages.

[0036] As shown in FIG. 7 in an alternate embodiment, the system mayinclude a flexible band 60 for secure attachment to a portion of apatient's body 14 (e.g., a head or chest). The band 60 includes fieldgenerator 62 and a reference sensor 64 that provides feedback to thesignal generator in the position detection unit 22. The positiondetection unit 22 is connected via communication lines 66 to theflexible band 60, and is connected via communication lines 68 to aflexible medical instrument 70 having a remote sensor at its tip 72.Because the medical instrument 70 is not rigid, the sensor should bepositioned sufficiently close to the tip of the instrument 70 to provideaccurate position detection and monitoring within the patient's body.The display 20 may indicate the relative orientation of the instrument70 on one or more images as shown.

[0037] As illustrated in FIGS. 8 and 9 the registration process involvestwo fundamental steps: 1) recording the scan images of a predeterminedorientation and 2) mapping the spacial orientation of the positiondetection system onto the recorded images. For example, the orientationsof the prerecorded images may be in the sagittal (y-z plane), coronal(x-z plane) and/or axial (x-y plane) as shown in FIG. 8. The images maybe digitally stored and the distance between each scanned image isrecorded, as are the relative orientations of each set of images.

[0038] In one embodiment, fiducial markers 80 are placed on thepatient's head 14 prior to scanning with the scanner 82. The markersthen appear on certain of the scanned images, and may be located by theposition detection system as shown in FIG. 9. Specifically, when eachmarker 80 is sequentially located, for example with the tip 48 of amedical instrument 16, the user signals the registration unit, such asvia a computer keyboard 84. The registration unit then scans eachrecorded digital image beginning from one corner until it locates theidentified marker. In other embodiments this may be achieved by havingthe images appear on the display 20 and having the user identify themarkers by using a keyboard or mouse. Once each of the markers have beenlocated using the position detection unit, the registration unitgenerates a mapping function to translate the position detection data(in i-j-k coordinates) to the stored image orientation data (in x-y-zcoordinates).

[0039] In other embodiments, the patient may be wearing an attachedreference unit, such as the headset 12, when the scan images arerecorded. Based on the predefined structure of the reference unit, theregistration unit may then automatically locate portions of thereference unit on the scanned images, thereby identifying theorientation of the reference unit with respect to the scanned images.Since the relative orientation of the field generator with respect tothe reference unit is known, the registration unit may then generate theappropriate mapping function. In further embodiments the surfaces of thepatient's skin may be tracked such as by a laser light pointer or amovable tip pointer that is biased in a forward direction. The trackedsurfaces may then be located on the stored images. In still furtherembodiments, the registration unit could be programmed to identifycharacteristic structures or features of the patient's body and therebyprovide fully automatic registration. For example, the system might,knowing the size and shape of a headset, identify where the headsetwould be placed on the patient's head, even though it does not appear onthe prerecorded images.

[0040] As discussed above the position detection system may operate byany desired principles suitable for generating a field in which positiondetection may be achieved at any location within the field. For example,applicants have found that the 3 Space® Fastrak™ product sold byPolhemus, Incorporated of Colchester, Vermont operates via principlessuitable for use in the present invention. This product uses threeorthogonally disposed coil loops for both the transmitter and thesensors, and produces alternating electromagnetic fields of 8-14 khzthat are time division multiplexed. Those skilled in the art willappreciate that the relative positioning of the field generator and theone or more field sensors is in no way limited to those shown in FIGS. 1and 7.

[0041] As shown in FIG. 10, in alternate embodiments of the invention areference unit 86, including a field generator 88, may be positioned asmall distance away from the portion of the patient's body (such as thehead) 14 on an articulated arm 90. A headset 12 including a referencesensor 92 may be attached to the patient's body, and the medicalinstrument 16 may include a remote sensor 40 as discussed above withreference to FIGS. 1-6. Once the field generator 88 is positioned at aconvenient location it may be fixed in place by securing the joints ofthe articulated arm. The position of the patient with respect to thefield generator may accordingly be monitored. The position of theinstrument 16 with respect to the patient may also be determined and thesystem may then operate to display the appropriate prerecorded images asdiscussed above.

[0042] In other embodiments, the position of the field generator 88 maybe adjusted during the surgical operation by moving the articulatedjoints. If neither the remote sensor 40 nor the reference sensor 92 aremoved with respect to one another, then moving the field generator 88should not affect the position detection system. If the accuracy of thesystem depends at all on the relative positions of the field generators88 and the sensors 40, 92, then it may be desirable to move the fieldgenerator 88 during the surgical operation. This may be the case, forexample, if the system relies on the near-field characteristics of amultiplexed magnetic field wherein it might be desirable to keep thesensors 40, 92 generally equidistant from the generator 88. In stillfurther embodiments, the system may periodically prompt the user toreposition the generator 88 such as through visual cues on the display.

[0043] The monitoring of the position of the patient may be accomplishedby means other than using a headset and reference sensor. For example, acamera 94 connected to an image processor 96 may be positioned to recordthe location of the field generator with respect to the target operationsite of the patient as shown in FIG. 11. If either the patient or thefield generator is moved, the image processor 96 will identify theamount of relative change in location and advise the position detectionunit 22 accordingly. Additional cameras positioned to view the patientfrom a variety of directions may be employed in further embodiments.

[0044] As shown in FIGS. 12 and 13 a system of the invention may includea flexible medical instrument 100 having a sensor 102 at its distal tip104, and a fiber optic endoscope 106 having a sensor 108 at it distaltip 110. The fiber optic endoscope 106 is connected at its proximal endto a camera 112 which is in communication with an image processor 114.Because the field generator 62 on the reference band 60 may move, forexample as the patient breaths, the location of the remote sensor 102may appear to move when in fact the medical instrument 100 has notmoved.

[0045] To correct for this problem, the fiber optic endoscope 106 can beused to monitor the position of the tip 104 of the instrument 100 withrespect to the inside of the patient's body as shown. Any sensedmovement of the sensor 102 with respect to the field generator 62 can beevaluated with reference to whether the tip 104 has moved with respectto the interior of the patient's body. If the camera observes that thetip 104 has not moved, but the sensor 102 indicates that it has moved,then the system can identify that such movement was due to the movementof the field generator and not the sensor 102. The system may thenautomatically correct for such variation. Further, the fiber opticendoscope 106 itself may include a sensor 108 for detecting whether thetip 110 of the fiber optic has moved. This should further enhance theaccuracy of the correction system. Also, the camera 112 may providecontinuous registration of the prerecorded images based on the internalstructure of the patient's body.

[0046] It will be understood by those skilled in the art that numerousvariations and modifications may be made to the above describedembodiments without departing from the spirit and scope of the presentinvention.

We claim:
 1. A system for monitoring the position of a medicalinstrument with respect to a patient's body and for displaying at leastone of a plurality of prerecorded images of said body responsive to theposition of said medical instrument, said system comprising: a referenceunit securable from movement with respect to a target operating site onsaid patient's body; a remote unit for attachment to said medicalinstrument; a field generator associated with one of said units forgenerating a position characteristic field in an area including saidtarget operation site; a field sensor associated with the other of saidunits responsive to the presence of said position characteristic fieldfor producing a sensor output signal representative of said sensedfield; position detection means responsive to said sensor output signalfor producing position data representative of the position of saidremote unit with respect to said reference unit; and output displaymeans in communication with said position detection means for displayingat least one of said prerecorded images responsive to said positiondata.
 2. A system as claimed in claim 1, wherein said reference unitincludes said field generator, and said remote unit includes said fieldsensor.
 3. A system as claimed in claim 2, wherein said reference unitfurther includes a reference sensor responsive to the presence of saidposition characteristic field for producing a reference sensor outputsignal representative of said sensed field, said reference sensor beingpositioned a fixed distance from said field generator.
 4. A system asclaimed in claim 1, wherein said reference unit is in contact with saidpatient's body.
 5. A system as claimed in claim 4, wherein saidreference unit includes a headset unit comprising two ear mountingelements for engaging the ear canal openings of said patient's body, anda nose mounting element for engaging the nose bridge of said patient'sbody.
 6. A system as claimed in claim 5, wherein each said mountingelement is disposed on the distal end of an elongated mounting arm.
 7. Asystem as claimed in claim 6, wherein said elongated mounting arms aremade of a structurally resilient material.
 8. A system as claimed inclaim 1, wherein said reference unit includes a flexible band forattachment to a portion of said patient's body and said band furtherincludes at least one reference sensor for producing a reference sensoroutput signal representative of the position detection field sensed bysaid reference sensor.
 9. A system as claimed in claim 8, wherein saidflexible band further includes said field generator.
 10. A system asclaimed in claim 1, wherein said medical instrument is an aspiratingdevice.
 11. A system as claimed in claim 1, wherein said medicalinstrument includes a flexible member for insertion into said body andsaid remote unit includes a field sensor at the distal end of saidflexible instrument.
 12. A system as claimed in claim 1, wherein saidposition characteristic field includes an electromagnetic field.
 13. Asystem as claimed in claim 1, wherein said field generator includesmeans for generating three mutually distinguishable orthogonallydisposed electromagnetic dipole fields, and said field sensor includesmeans for sensing said mutually distinguishable electromagnetic dipolefields.
 14. A system as claimed in claim 1, wherein said system furtherincludes registration means for registering the spacial orientation ofsaid prerecorded images with the spacial orientation of said positioncharacteristic field generated by said field generator.
 15. A system asclaimed in claim 14, wherein said registration means includes; markeridentification means for identifying on said prerecorded images aplurality of registration markers and producing a first set ofcoordinates thereby; detection means for determining the position ofsaid markers with respect to said field generator and producing a secondset of coordinates thereby; and mapping means for mapping one of saidsets of coordinates onto the other.
 16. A system as claimed in claim 14,wherein said registration means includes reference unit identificationmeans for identifying on said prerecorded images the orientation of saidreference unit with respect to said images.
 17. A system as claimed inclaim 1, wherein said output display means further includes means fordisplaying a plurality of images at the same time.
 18. A system asclaimed in claim 1, wherein said output display means further includesmeans for displaying on said displayed prerecorded image an indicatorrepresentative of the location of said medical instrument with respectto said patient's body responsive to said position data.
 19. A system asclaimed in claim 1, wherein said prerecorded image is displayed on saidoutput display means as a portion of a composite three dimensionaldisplay.
 20. A system as claimed in claim 1, wherein said output displaymeans further includes means for displaying on said displayedprerecorded image an indicator representative of the location andrelative orientation of said medical instrument with respect to saidpatient's body responsive to said position data.
 21. A system formonitoring the position of a medical instrument with respect to apatient's body and for displaying at least one of a plurality ofprerecorded images of said body responsive to the position of saidmedical instrument, said system comprising: a reference unit including afield generator for generating a position characteristic field in anarea including a target operation site, a remote unit for attachment tosaid medical instrument, said remote unit including a field sensorresponsive to the presence of said position characteristic field forproducing a sensor output signal representative of said sensed field;patient tracking means for monitoring the position of said patient withrespect to said reference unit and for producing a position adjustmentsignal; medical instrument tracking means for in communication with saidsensor output signal for producing position data representative of theposition of said remote unit with respect to said reference unit; datacorrection means for adjusting said position data responsive to saidposition adjustment signal and producing adjusted position data; andoutput display means in communication with said position detection meansfor displaying at least one of said prerecorded images responsive tosaid adjusted position data.
 22. A system as claimed in claim 21,wherein said patient tracking means includes a reference sensor securedfrom movement with respect to at least a portion of said patient's bodyin the area of said target operation site, said reference sensor forproducing said position adjustment signal.
 23. A system as claimed inclaim 21, wherein said patient tracking means includes image recordingmeans for recording images and image processing means for producing saidposition adjustment signal responsive to said recorded images, whereinat least said reference unit and a portion of said patient's body arewithin view of said image recording means.
 24. A system as claimed inclaim 21, wherein said field generator is mounted on an articulated arm.25. A system as claimed in claim 22, wherein said reference sensor isattached to a headset unit comprising two ear mounting means forengaging the ear canal openings of said patient's body, and nosemounting means for engaging the nose bridge of said patient's body. 26.A system as claimed in claim 25, wherein each said mounting element isdisposed on the distal end of an elongated mounting arm.
 27. A system asclaimed in claim 26, wherein said elongated mounting arms are made of astructurally resilient material.
 28. A system as claimed in claim 21,wherein said medical instrument is an aspirating device.
 29. A system asclaimed in claim 21, wherein said position characteristic field includesan electromagnetic field.
 30. A system as claimed in claim 21, whereinsaid field generator includes means for generating three mutuallydistinguishable orthogonally disposed electromagnetic dipole fields, andsaid field sensor includes means for sensing said mutuallydistinguishable electromagnetic dipole fields.
 31. A system as claimedin claim 21, wherein said system further includes registration means forregistering the spacial orientation of said prerecorded images with thespacial orientation of said position characteristic field generated bysaid field generator means.
 32. A system as claimed in claim 31, whereinsaid registration means includes; marker identification means foridentifying on said prerecorded images a plurality of registrationmarkers and producing a first set of coordinates thereby; detectionmeans for determining the position of said markers with respect to saidfield generator and producing a second set of coordinates thereby; andmapping means for mapping one of said sets of coordinates onto theother.
 33. A system as claimed in claim 31, wherein said registrationmeans includes reference unit identification means for identifying onsaid prerecorded images the orientation of said reference unit withrespect to said images.
 34. A system as claimed in claim 21, whereinsaid output display means further includes means for displaying aplurality of images at the same time.
 35. A system as claimed in claim21, wherein said output display means further includes means fordisplaying on said displayed prerecorded image an indicatorrepresentative of the location of said medical instrument with respectto said patient's body responsive to said position data.
 36. A system asclaimed in claim 21, wherein said prerecorded image is displayed on saidoutput display means as a portion of a composite three dimensionaldisplay.
 37. A system as claimed in claim 21, wherein said outputdisplay means further includes means for displaying on said displayedprerecorded image an indicator representative of the location andrelative orientation of said medical instrument with respect to saidpatient's body responsive to said position data.
 38. A method ofmonitoring the position of a medical instrument with respect to apatient's body and displaying at least one of a plurality of prerecordedimages of said body responsive to the position of said medicalinstrument, said method comprising the steps of: generating a positioncharacteristic field in an area including a target operation site ofsaid patient's body; sensing a portion of said field and producing asensor output signal representative of said portion of said field;generating tracking reference data responsive to said sensor outputsignal representative of the position of a remote unit attached to saidmedical instrument with respect to a reference unit secured frommovement with respect to at least a portion of said patient's body inthe vicinity of said target operation site; and displaying a desiredprerecorded image on an output display unit responsive to said trackingreference data.
 39. A method of monitoring the position of a medicalinstrument with respect to a patient's body and displaying at least oneof a plurality of prerecorded images of said body responsive to theposition of said medical instrument, said method comprising the stepsof: storing said plurality of prerecorded images of said body, each saidprerecorded image being representative. of a planar region within saidbody such that the plurality of planar regions represented by saidprerecorded images define a first coordinate system; generating aposition characteristic field in an area including a target operationsite of said patient's body; inserting at least a portion of saidmedical instrument into said patient's body; sensing a portion of saidfield and producing a sensor output signal representative of saidportion of said field; generating tracking reference data with respectto a second coordinate system representative of the position of a remoteunit attached to said medical instrument with respect to a referenceunit secured from movement with respect to at least a portion of saidpatient's body in the vicinity of said target operation site, saidreference data being generated responsive to said sensor output signal;correlating said tracking reference data with said plurality ofprerecorded images and identifying a desired prerecorded imageassociated with the position of said remote unit with respect to saidpatient's body; and displaying said desired prerecorded image on anoutput display unit responsive to said tracking reference data. 40.Apparatus for securing a reference unit to a patient's head, saidapparatus comprising reference unit mounting means for mounting saidreference unit to said apparatus, said reference unit for use inmonitoring the position of a medical instrument with respect to saidpatient's head, and a nose bridge mounting element and two ear mountingelements for attaching said reference unit to said patient's head. 41.Apparatus as claimed in claim 40, wherein said reference unit mountingmeans includes means for releasably attaching said reference unit tosaid apparatus.
 42. Apparatus as claimed in claim 40, wherein each ofsaid nose and ear mounting elements are disposed on a distal end of anelongated mounting arm.
 43. Apparatus as claimed in claim 40, whereinsaid apparatus further includes an elongated center member and two sidemembers, each of said members being centrally attached to one another atfirst ends thereof, and said nose bridge mounting element being attachedto said center member at a second end thereof and each of said earmounting elements being attached to a respective side member at secondends thereof.
 44. Apparatus as claimed in claim 42, wherein each of saidcenter and side members is made of a structurally resilient material andis capable of accommodating a plurality of sizes of patient heads. 45.Apparatus as claimed in claim 44, wherein said structurally resilientmaterial is plastic.
 46. Apparatus as claimed in claim 40, wherein saidreference unit includes a field generator for generating a threedimensional position characteristic field.
 47. Apparatus as claimed inclaim 46, wherein said reference unit further includes a referencesensor for generating a feedback reference signal.
 48. Apparatus asclaimed in claim 40, wherein said reference unit includes anelectromagnetic field generator for generating an electromagnetic field.49. Apparatus as claimed in claim 40, wherein said reference unitincludes a field sensor for sensing a three dimensional positioncharacteristic field and for producing a sensor output responsive to thepresence of said sensed field.
 50. Apparatus as claimed in claim 49,wherein said reference unit further includes a reference sensor forgenerating an error detection signal.
 51. Apparatus as claimed in claim40, wherein said reference unit includes an electromagnetic field sensorresponsive to the presence of an electromagnetic field for producing asensor output signal representative of said sensed electromagneticfield.
 52. Apparatus as claimed in claim 40, wherein said reference unitincludes a signal transmitter for transmitting a reference signal to asignal receiver, said reference signal for use in detecting the positionof said reference unit with respect to said signal receiver by comparingsaid signal to a second transmitted signal.
 53. Apparatus as claimed inclaim 50, wherein said reference unit further includes a second signaltransmitter for transmitting said second transmitted signal. 54.Apparatus as claimed in claim 40, wherein said reference unit includes asignal receiver for receiving a reference signal from a signaltransmitter, said reference signal for use in detecting the position ofsaid reference unit with respect to said signal transmitter by comparingsaid signal to a second transmitted signal.
 55. Apparatus as claimed inclaim 54, wherein said reference unit further includes a second signalreceiver for receiving said second transmitted signal.
 56. Apparatus formonitoring the position of a medical instrument with respect to apatient's body, said apparatus including a reference unit removablyattached to said medical instrument.
 57. Apparatus as claimed in claim56, wherein said reference unit is removably insertable into saidmedical instrument.
 58. Apparatus as claimed in claim 56, wherein saidmedical instrument is an aspirating device.
 59. Apparatus as claimed inclaim 56, wherein said reference unit includes a field generator forgenerating a three dimensional position characteristic field. 60.Apparatus as claimed in claim 56, wherein said reference unit furtherincludes a reference sensor for generating a feedback reference signal.61. Apparatus as claimed in claim 56, wherein said reference unitincludes an electromagnetic field generator for generating anelectromagnetic field.
 62. Apparatus as claimed in claim 56, whereinsaid reference unit includes a field sensor for sensing a threedimensional position characteristic field and for producing a sensoroutput responsive to the presence of said sensed field.
 63. Apparatus asclaimed in claim 56, wherein said reference unit further includes areference sensor for generating an error detection signal.
 64. Apparatusas claimed in claim 56, wherein said reference unit includes anelectromagnetic field sensor responsive to the presence of anelectromagnetic field for producing a sensor output signalrepresentative of said sensed electromagnetic field.
 65. Apparatus asclaimed in claim 56, wherein said reference unit includes a signaltransmitter for transmitting a reference signal to a signal receiver,said reference signal for use in detecting the position of saidreference unit with respect to said signal receiver by comparing saidsignal to a second transmitted signal.
 66. Apparatus as claimed in claim65, wherein said reference unit further includes a second signaltransmitter for transmitting said second transmitted signal. 67.Apparatus as claimed in claim 56, wherein said reference unit includes asignal receiver for receiving a reference signal from a signaltransmitter, said reference signal for use in detecting the position ofsaid reference unit with respect to said signal transmitter by comparingsaid signal to a second transmitted signal.
 68. Apparatus as claimed inclaim 67, wherein said reference unit further includes a second signalreceiver for receiving said second transmitted signal.